Structure of the NGC 1023 System

V.L. AJanasiev 1, M. Capaccio/i 2, H. Lorenz 3, and G.M. Richter 3 ISpecial Astrophysical Observatory, SU-357140 Zelenchukskaja, USSR 2Institute of Astronomy, University of Padova, 1-351000 Padova, Italy 3Central Institute for Astrophysics, DDR-1561 Potsdam, German Democratic Republic

ABSTRACT: The controversial origin of the neutral hydrogen cloud in the vicinity of the SO NGC 1023 is re-discussed on the basis of new optical data. We assume that NGC 1023A has experienced a tidal interaction with NGC 1023. The model explains some of the pecularities of NGC 1023A, but is probably unable to account for all of the HI mass about NGC 1023.

1. Introduction The SBO galaxy NGC 1023 is the brightest member of the group of rich in late-type spirals and dwarf galaxies and was discovered to be one of the nearest objects of a rare class of neutral-hydrogen rich lenticular galaxies (van Woerden, 1977). Studies at increasing sensitivity and resolution led to the pic• ture of the HI distribution given by Sancisi et al. (19B4). NGC 1023 appears surrounded by 1.5 109 Mo of neutral hydrogen with a rather complex density distri• bution and kinematics. Two clumpy ring-like cloudes of HI with different mean sys• temic velocities are identified, carrying almost the same amount of material and showing an overall rotation in the same sence as the stellar body, although in different planes. The "red cloud" has a strong condensation superposed on the op• tical image pf NGC 1023A and it seems morphologically and kinematically linked to a "tail" at higher .

2. NGC 1023 / NGC 1023A an interacting system The interpretation of this picture is closely connected to the nature of HI sur• rounding early-type galaxies. This material is not evidently associated with re• gions of formation, tends to be distributed mainly outside the optical bound• aries and shows a fairly regular rotation with momenta not always aligned with those of the stellar bodies. The main question is: accretion at the expense of intergalactic HI cloudes or dwarf galaxies or remnants of previous disks mainly consumed by earlier star formation.

NeC 1023A has been investigated in detail by Capaccioli et al. (19B6). The basic data on NGC 1023 and NGC 1023A are summarized in their tables 4 and 5.

Dynamics and Interactions or Galaxies Edilor:R. Wielen 91 © Springer-Verlag Berlin, Heidelberg 1990 The classification of NGC 1023A as a Magellanic irregular or a low surface brighiness late-type dwarf, consistent with the absolute magnitude and with the linear size of the object, is in contrast with the quite smooth texture of the galaxy image, with the lack of emission lines, with the colours to be far too red and with the low content of HI. The proximity of NGC 1023A to NGC 1023 as well as the provocativ morphology of the neutral hydrogen in the area of NGC 1023 (par• ticularly the "tail") suggest a possible way to remove the discrepancies. Following the general scenario developed by Sancisi et al. (1984) we assume that NGC 1023A has experienced a tidal interaction with the more massive galaxy NGC 1023 about 108 yr ago. Although insufficiently strong to disrupt the galaxy, the fly-by encounter may have caused the removal of a large fraction of the inter• stellar material of NGC 1023A. The consequences of that are a cut of the value of MHI/L8 and a sudden reduction of the star formation activity, which may account for the observed lack of structures and of emission lines in the galaxy and for their red colours. If we take the ratio of all the HI mass around NGC 1023 to the present luminos• ity of NGC 1023A, we obtain MHI/La=5, a large value at the upper boundary of the distribution of dwarf galaxies in the La vs. MHI/La plane. The agreement improves but not significantly if we increase the a-luminosity before the encounter even by 0.5 mag., but we have to consider the effect of the fragmentation of the stellar body during the encounter. This is difficult to quantify, but the lack of major optical debris indicates that the correction to the luminosity is likely to be small. Thus, although not totally inconsistent with the data, it does not seem likely that all of the HI around NGC 1023 has been supplied by the dwarf galaxy: This supports the pressumption first indicated by Sancisi et al. (1984) that NGC 1023A is probably the donor of just the HI "tail" and the "red cloud".

References Capaccioli, M., Lorenz, H. and Afanasjev, V. L.: 1986, Astron. Astrophys. 169, 54. Sancisi, R., van Worden, H., Davies, R. D., Hart, L.: 1984, Monthly Notices Roy. Astron. Soc. 210, 514. van Woerden, H.: 1977, Topics in Interstellar Matter, ed. van Woerden, Reidel, Dortrecht, p. 261.

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